Gas recirculation duct



y 1, 1963 H. E. BURBACH ETAL 3,090,332

GAS RECIRCULATION DUCT 2 Sheets-Sheet 1 Filed Dec. 29, 1960 hl llppuhllwlilrlr with. illvnllllhh hlp up INVENTORS: HENRY E. BURBACH OTTO T. HIPPATTORiEY y 1, 1963 H. E. BURBACH ETAL 3,090,332

GAS RECIRCULATION DUCT Filed Dec. 29, 1960 2 Sheets-Sheet 2 INVENTORS:HENRY E. BURBACH OTTO T. HIPP BY Q ATTORNEY United States Patent Ofiice3,999,332 Patented May 21, 1963 3,ii90,332 GAS RECIRCULATION DUCT HenryE. Burbach, Avon, and fltto T. Hipp, West Hartford, Conn, assignors toCombustion Engineering, Inc, Windsor, Conn, a corporation of DelawareFiled Dec. 29, 1960, Ser. No. 79,356 2 Claims. (Cl. 110-49) Thisinvention relates to steam generating units employing gas recirculation,where a portion of the gases leaving the furnace are recirculated backto the furnace for temperature control.

In modern steam generating plants, it is found that as the loadrequirements decrease, the steam temperature also decreases, and this isnot desirable. To obtain greatest turbine efliciency, the steam shouldbe kept at a constant temperature, regardless of the load requirements.One of the methods of maintaining constant steam temperature is by gasrecirculation. As the load decreases, more combustion gas isrecirculated back to the furnace. This prevents to a large extent theamount of heat that passes from the combustion gases within the furnaceby radiation to the tubes lining the furnace walls, and hence leaves agreater amount of heat in the gases leaving the furnace outlet to beabsorbed by the convection heat exchange members, such as thesuperheater and reheater. Also, a greater volume of gas leaves thefurnace when recirculated gas is introduced into the furnace. In thismanner, the steam temperature is maintained substantially constant,regardless of load changes.

This invention is particularly directed to the construction and positionof the gas recirculation duct extending between the gas pass downstreamof the heat exchange means and the furnace, and especially in a coalfired furnace. In a coal fire furnace, the hot combustion gases leavingthe furnace contain a large amount of dust, fly ash and other foreignparticles. In present steam generating units, dust collectors areusually installed in the recirculating gas ducts, on coal fired units,upstream of the fan to remove the dust and ash therefrom. If it is notremoved, serious problems can result due to the large amount of such ashand dust being reintroduced into the furnace along with the recirculatedgas.

In accordance with this invention, a portion of the recirculated gasduct is contained within the gas pass, and the openings thereto are onthe underside, such that the gases entering must pass through a 180turn. By such construction, a large portion of the fly ash, dust, andother foreign particles are prevented from entering the recirculated gasduct. This also gives protection to the fan in the recirculated gasduct, the abrasive action of the fly ash and dust on the fan being verydamaging and costly.

Other advantages are that the duct within the gas pass need not beinsulated. The temperature of the gases in the gas pass dovtmstream ofthe heat exchange members is approximately 700 F., and normally theentire gas recirculation duct must be insulated. Also, positioning aportion of the duct within the gas pass results in a substantial savingof space. In large modern boilers this is of significance. Anotheradvantage is that only one opening through the wall of the gas pass mustbe sealed, this being the opening through which the recirculated gasduct extends. In ordinary unit-s utilizing gas recirculation, two ormore openings are usually provided for attachment of gas recirculationducts, which connections must be suitably sealed. The above listedadvantages apply not only to a coal fired furnace, but apply equallywell to any furnace, regardless of the type of fuel being burned.

Additional objects and advantages of the invention will appear from thefollowing description of a preferred embodirnent thereof when consideredin conjunction with the accompanying drawings wherein:

FIGURE 1 is a sectional side elevation of a furnace employing gasrecirculation;

FIGURE 2 is a view of the furnace taken along lines 22 of FIGURE 1.

Referring to FIGURE 1 of the drawings, 1d designates a horizontalcyclone fired furnace utilizing pulverized coal as a fuel. The walls ofthe furnace are lined with tubes 12, which absorb heat radiantly fromthe furnace gases to produce steam. These tubes 12 are supplied withwater from lower headers 13. Pulverized coal along with air isintroduced into the furnace through horizontal cyclone burners 14, theair being supplied by duct 16. This air is heated in air preheater 18prior to introduction into the furnace. The furnace 10 contains a hopperbottom 20, the walls of which are also lined with tubes 12. Below thefurnace is an ash compartment 22, into which the ash from the furnace isallowed to fall.

The hot combustion gases within the furnace rise upwardly past thenarrow portion of the furnace formed by arch 24, then flow through ahorizontal pass (shown schematically) and flow downwardly throughvertical gas pass 26. Gas pass .26 contains convectionsteam heatingmeans 28, such as a superheater and reheater, where a substantialportion of the heat of the hot combustion gases is withdrawn therefrom.The gases continue on past economizer 3i where more of the heat isutilized, and then flows on to air preheater 18 by means of duct 32.

A portion of the gases are withdrawn from gas pass 26 and arerecirculated back to the furnace as the need arises. For this purpose aduct 34 extends into gas pass 26, and contains openings 31, 35, and 36on the underside thereof by means of which the gases are allowed toenter the duct. The duct 34 is connected to duct 33 outside of the gaspass 26, which duct 33 passes recirculated gas to fan 39 by means ofducts 38. Manifold or duct 41 receives the discharge of fan 39.Extending transversely from manifold 41 are a plurality of ducts 42,which are connected to openings 44 in the bottom or throat of hopper 20.The tubes .12 are staggered, as shown at 46 in FIGURE 1, to allowentrance of the recirculated gas therebe-tween.

Duct 33 contains damper 37, by means of which the amount of recirculatedgas passing back to the furnace can be regulated. This damper can beadjusted manually or automatically, as is well known in the art, toincrease the amount of gas recirculation as the load requirementsdecrease, to thereby maintain a constant steam temperature.

As illustrated in FIGURE 2, the openings 31, 35, and 36 aresubstantially evenly spaced across the entire width of gas pass 26.Also, duct 34 is centrally located within the gas pass 26. This preventsan imbalance of gas flow through the heat exchange means 28 and 30directly above duct 34, due to the suction created by fan 39 at openings31, 35, and 36. Since the suction effect will be greatest near the leftend, the openings can be made progressively larger, openings 36 beingthe largest. Duct 34 is supported directly beneath economizer 30 bymeans of I-bearns 48, which can also be used to support the economizer.A circular or sloping contour at the top of duct 33 presents astreamlined surface, thereby insuring a minimum resistance to gas flow,and thus very little draft loss, as well as achieving a self-cleaningsurface.

From the foregoing it can be seen that any gases to be recirculated backto the furnace must pass through a turn to enter openings 31, 35, and 36in duct 34. Due to the velocity of the flow through gas pass 26, and theforce of gravity, very little fly ash, dust, or other foreign particleswill be drawn into openings 31, 35, and as :by the suction created byoperation of fan 39. The sloping or circular top edge of duct 33 willtend to force the fly ash and dust impinging thereon away from the duct,thus preventing it from entering openings 31, 35, and 36. Insummarizing, it can be seen that fly ash carryover to the gasrecirculation system is reduced simply and efiiciently, resulting inreduction of fan maintenance costs and reduction of reintroduction ofsuch fly ash into the furnace. Also, the unit is compact and economical,there being no need for insulation of the duct within the gas pass.

While the preferred embodiment of the invention has been shown anddescribed, it will be understood that such is merely illustrative andnot restrictive.

What we claim is:

1. A steam generating unit comprising a furnace, fuel and air inlets forsaid furnace, an outlet leading from the upper portion of said furnacethrough which the hot combustion gases pass, vertical wall means forminga gas pass containing heat exchange means, said gas pass being connectedto said furnace outlet such that the gases pass downwardly therethrough,a duct in communication at one end with the gas pass downstream of saidheat exchange means, and in communication at its other end with saidfurnace, a fan in said duct, a portion of said duct contained withinsaid gas pass, said portion of said duct extending across the centerportion of the gas pass, a plurality of openings in said duct beingevenly spaced across the width of the gas pass through which gases areallowed to flow from the gas pass into said duct, said openings beingpositioned on the underside of said duct, such that the gases flowing inthe gas pass must make a 180 turn to enter said openings, said openingsprogressively decreasing in size, the smallest opening being located theclosest to the fan, so there is no imbalance of flow of gases throughthe portion of the gas pass containing said heat exchange means due tothe suction effect of said fan. i

2. In a steam generating unit, a furnace, fuel and air inlets for saidfurnace, an outlet leading from said furnace through which the hotcombustion gases pass, Wall means forming a gas pass containing heatexchange means, said gas pass being connected to said furnace outlet, aduct in communication at one end with the gas pass downstream of saidheat exchange means, and in communication at its other end with saidfurnace, a fan in said duct, a portion of said duct contained within thegas pass, said portion of said duct extending across the center portionof the gas pass, a plurality of openings in said duct through whichgases are allowed to flow from the gas pass into said duct, saidopenings being positioned such that the gases flowing in the gas passmust make a 180 turn to enter such openings, thereby preventing ash orother foreign particles carried by said gases from entering the duct,said openings progressively decreasing in size, the smallest openingbeing located the closest to the fan, so there is no imbalance of flowof gases through the portion of the gas pass containing said heatexchange means due to the suction effect of said fan.

References Cited in the file of this patent FOREIGN PATENTS 744,797Great Britain Feb. 15, 1956

1. A STEAM GENERATING UNIT COMPRISING A FURNACE, FUEL AND AIR INLETS FORSAID FURNACE, AN OUTLET LEADING FROM THE UPPER PORTION OF SAID FURNACETHROUGH WHICH THE HOT COMBUSTION GASES PASS, VERTICALL WALL MEANSFORMING A GAS PASS CONTAINING HEAT EXCHANGE MEANS, SAID GAS PASS BEINGCONNECTED TO SAID FURNACE OUTLET SUCH THAT THE GASES PASS DOWNWARDLYTHERETHROUGH, A DUCT IN COMMUNICATION AT ONE END WITH THE GAS PASSDOWNSTREAM OF SAID HEAT EXCHANGE MEANS, AND IN COMMUNICTION AT ITS OTHEREND WITH SAID FURNACE, A FAN IN SAID DUCT, A PORTION OF SAID DUCTCONTAINED WITHIN SAID GAS PASS, SAID PORTION OF SAID DUCT EXTENDINGACROSS THE CENTER PORTION OF THE GAS PASS, A PLURALITY OF OPENINGS INSAID DUCT BEING EVENLY SPACED ACROSS THE WIDTH OF THE GAS PASS THROUGHWHICH GASES ARE ALLOWED TO FLOW FROM THE GAS PASS INTO SAID DUCT, SAIDOPENINGS BEING POSITIONED ON THE UNDERSIDE OF SAID DUCT, SUCH THAT THEGASES FLOWING IN THE GAS PASS MUST MAKE A 180* TURN TO ENTER SAIDOPENINGS, SAID OPENINGS PROGRESSIVELY DECREASING IN SIZE, THE SMALLESTOPENING BEING LOCATED THE CLOSEST TO THE FAN, SO THERE IS NO IMBALANCEOF FLOW OF GASES THROUGH THE PORTION OF THE GAS PASS CONTAINING SAIDHEAT EXCHANGE MEANS DUE TO THE SUCTION EFFECT OF SAID FAN.